The present invention relates to transparent polymeric materials containing sub-micron to nano size bubbles which demonstrate low density, high fracture toughness, high fracture strain, and lower haze than prior art similar such materials, and to a method for their fabrication.
Current monolithic and laminated canopies and windscreens for fighter aircraft and the like are fabricated using PMMA (polymethyl methyl methacrylates) acrylics and bisphenol polycarbonates. These materials have relatively low use temperature limits, up to about 220xc2x0 F. and 300xc2x0 F. for acrylics and polycarbonates respectively. Wind tunnel testing of such structures conducted at Mach 1.6 to 3.0 at specific altitudes and exposure times showed that the transparency surface temperatures varied from 200xc2x0 F. to 500xc2x0 F. Consequently, there is a clearly perceived need for improved transparent materials, which demonstrate a higher temperature capability. Concurrently, in such applications, structural performance, optics (transparency and the avoidance of multiple imaging), and numerous other demanding capabilities are required of the material. As aircraft are produced to fly even faster and under more stringent conditions, these demands will all increase.
In the case of commercial aircraft, acrylics and polycarbonates are commonly used for subsonic canopies, windows and windscreens, since temperature is generally not an issue in such service. However, in the case of supersonic commercial aircraft, the demands will be very much the same as those for military aircraft. Additionally, due to thermal instability in a fire situation, windows of subsonic commercial aircraft tend to pop out, thus allowing air (oxygen) and flame to enter the cabin area more readily. The use of higher temperature capability transparent polymeric materials in these applications, is accordingly also desirable.
Recently, methods have been developed for the production of Tg (202-350xc2x0 C.) transparent polymers. Much of this work has involved the synthesis of new high Tg polymers or polymer blends such as blends of polyetherketone with polyethersulfone, miscible sulfonated polyetheretherketone with polyetherimide, melt blends of phenylene ether phosphine oxide based on hydroquinone and bisphenol with polyeteretherketone, synthesis of aromatic polybenzoxazoles in trimethylsilyl polyphosphate, and 6F-polybenzoxazoles to obtain adequate transparency, structural capacity and temperature capability. A particularly interesting class of such materials and the methods of their synthesis are described in U.S. Pat. No. 5,691,442 to Unroe, et al issued November 1997 which is incorporated herein by reference. This patent describes improving the transparency of poly(arylene ether) homopolymers or copolymers by endcapping the polymeric chain with an unsubstituted phenolic-based endcapping agent. Such materials, when cast as thin films from chloroform, yield tough, transparent and colorless films. The Tg of these materials is in the range of 207xc2x0 C. and 281xc2x0 C., their tensile strengths are in the range of 10.4-12.7 ksi, their tensile modulus in the range of 0.26 and 0.37 msi and their fracture strains in the range of 4% to 58%. When processed as thick sheet by injection molding or compression molding, however, the materials become yellow thus limiting applications as described above, that require good transparency.
Whatever the material used to fabricate canopies according to the prior art, such as thermal forming, injection molding, etc. the resulting structure was a xe2x80x9csolidxe2x80x9d material having the density of the parent polymeric material. Additionally, the thermal processing of xe2x80x9cthickxe2x80x9d sections of high Tg polymers, of the type required for the aforementioned applications, generally produces an undesirable coloration as the thickness of the structure increases, as in the case of the polymers of Unroe et al. Alternatively, a complex and time consuming laminating process must be used to obtain structures, which demonstrate the required clarity.
The materials of the present invention, produced in accordance with the process of the present invention, contain up to about 30% voids and demonstrate highly desirable optical characteristics that are required for the above described applications while being significantly lighter, i.e. about 16% lighter because of their foamed structure.